This invention relates to composite film materials and methods for their preparation. More particularly it relates to composite films and methods of manufacture wherein highly crystalline polymeric materials (e.g., those having a mass crystallinity greater than about 60%) are non-adhesively bonded to polyester film, even though said polymeric materials have heretofore been essentially non-bondable to polyester. As used herein the term "mass crystallinity" means the mass fraction (in percent) of a material that is crystalline (Encyclopedia of Polymer Science and Technology, Vol. 4, pp 472 (1966).
Multilayer composite film structures which employ non-adhesive bonding of their various layers are known. Thus see U.S. Pat. Nos. 3,188,265 and 3,188,266. These patents describe two layer composite film structures having a polyester film layer and a polyethylene film layer. The structures are prepared by laminating the two film layers together heating and then irradiating the laminate with electromagnetic radiation (e.g., ultraviolet light) in the wavelength range of about 1800-4000 angstroms. After irradiation the two layers are self-unified and are not mechanically separable from each other.
U.S. Pat. No. 3,188,266 further discloses that a third layer may be incorporated into the composite structure. The third layer comprises a solid body which has one of its surfaces adherently bonded to the polyethylene layer. The resulting structure can then be employed in an adhesive tape or sheet, a coated abrasive, a coated fabric or other coated or laminated polymer construction.
While the two and three layer constructions disclosed in these patents have proven to be useful in bonding polymers having a mass crystallinity of less than about 60% to polyester, it was discovered that those polymeric materials having a mass crystallinity of more than about 60% (sometimes referred to hereinafter as highly crystalline materials) did not form strong bonds to polyester when bonded by the technique described therein.
Consequently other techniques were employed in order to bond these materials to polyester. These techniques included, for example, either (i) priming the polyester surface or (ii) treating the polyester surface with corona discharge before joining the two materials together. They also included the use of adhesives to bond the materials to each other. However, none of these techniques has proven entirely satisfactory. Often the resulting bond between the materials was not very strong with the result that they frequently delaminated under even minimal stress conditions.
Additionally these techniques required the use of special equipment and/or processing steps. Still further the primers and adhesives often contained solvents which presented environmental, health and safety problems.
Moreover, the utility of certain of these composite film structures was limited. Such structures are often used in food packaging. Thus migrating solvents present in the primers and adhesives were undesirable. If the solvents could be removed from such composite structures, however, then the potential health and safety problems attendant with their use would be eliminated.
Coextrusion techniques have also been employed to bond highly crystalline materials to polyester. In such techniques, the two polymers are coextruded onto each other and the two layer structure then irradiated with ultraviolet light. However, the two layers typically have not adhered well to each other.